The present invention relates to a time division exchange and, more particularly, to a time division exchange for which a loop-back test can be readily carried out.
In a time division exchange, a time division multiplexed trunk is provided between a time division switching network and a time division multiplexed line. This time division multiplexed trunk converts signal levels between the time division multiplexed line and the switching network, realizes bipolar and unipolar conversion and also controls phase matching.
FIG. 1 is a block diagram of an essential part of a time division exchange. Subscriber circuits 9, 10 detect the ON-Hook and OFF-Hook state of subscribers A and B, and this detected information is transferred to a central controller (CC) 2 via a control signal receiving memory (SRM) 7 and a signal receiving distributor (SRD) 4. Dial signals are sent from the subscribers A and B, and are transferred to the central controller 2 through the control signal receiving memory 7 and the signal receiving distributor 4. When it is determined that a dial signal indicates a call to a subscriber in another (i.e. remote) exchange as a result of analyzing the dial signal, a time division multiplexed trunk 12 is activated and a start signal is sent to the remote exchange in accordance with a specified time slot on a time division multiplexed line 19. A response signal is generated by the remote exchange in response to the start signal. The response signal is transferred in such a form that it is inserted in a specified time slot on a time division multiplexed line 20, so that it is received and detected by the time division multiplexed trunk 12 and is then sent to the central controller 2.
FIG. 2(a) is a diagram of a sending frame format for the time division multiplexed lines 19, 20, in which F is a frame synchronous signal; D1, D2, . . . represents data, such as a voice PCM signal; C1, C2, . . . are control signals such as a start signal, a response signal, etc.; and one frame is composed of, for example, 24 channels, CH1 to CH24.
The waveform (b) in FIG. 2 is an example of a control signal sent from the time division multiplexed trunk 12 on the calling side, while the waveform (c) is an example of a control signal sent to the time division multiplexed trunk 12 from the called side. The control signals of FIG. 2(b) and FIG. 2(c) correspond respectively to the control signals C1 and C2, shown in FIG. 2(a). For example, if a subscriber A originates a call, the central controller 2 receives and accumulates the dial signal sent from the subscriber A. When it is discriminated, as a result of analysis, that this call must be routed to a subscriber accommodated by another exchange, the central controller 2 activates the time division multiplexed trunk 12. A start command is written in a control signal sending memory (SSM) 6 through the signal receiving distributor 4. The content of the control signal sending memory 6 is inserted in a specified time slot and is transferred to the time division multiplexed trunk 12 from the network 1 through a digital line concentrator 11. The time division multiplexed trunk 12 separates the specified time slot (and the information inserted therein) in an interface circuit 13, and transfers it to a control circuit 14. When the control circuit 14 discriminates the start command, it sends a start signal to the remote exchange. When this start signal is sent at the timing t1 shown in FIG. 2(b) to the time division multiplexed line 19, for example, for channel CH1, the remote exchange sends the control signal C1 of channel CH1, as the response signal, on the time division multiplexed line 20.
When the time division multiplexed trunk 12 receives the response signal at the timing t2 as shown in FIG. 2(c), it is transferred to the control circuit 14 through the interface circuit 13. The control circuit 14 determines that it is a response signal for channel CH1, sent from the remote exchange in response to the start signal for channel CH1 sent on the time division multiplexed line 19, and sends it to the central controller 2 through the control signal receiving memory 7 and the signal receiving distributor 4. The central controller 2 controls the sending of dial signals in accordance with this response signal. For example, the dial signal may be transmitted at the timing t3 (FIG. 2(b)). The remote exchange receives the dial signal and calls the designated subscriber. When it detects an answer from the designated (i.e., called) subscriber, the remote exchange sends an answer signal corresponding to the designated subscriber. When the time division multiplexed trunk 12 receives the answer signal at the timing t4 (FIG. 2 (c)), the received answer signal is transferred to the control circuit 14 through the interface circuit 13. The control circuit 14 determines that it is an answer signal of the called subscriber, and it informs the central controller 2 of the reception of the answer signal through the control signal receiving memory 7 and the signal receiving distributor 4. The central controller 2 controls the network 1 by writing speech path information into the control memory 5, thereby causing it to establish a speech path between the calling subscriber A and the called subscriber accommodated by the other exchange.
In the type of switching equipment illustrated in FIG. 1, testing is essential in order to check for normal exchanging operations, normal operation of trunk circuits, etc. As in the case of a space division type exchange, when it is required to test the structure, including the network 1 and the time division multiplexed trunk 12, in the time division exchange, testing for a normal exchanging connection is performed by connecting the time division multiplexed lines 19, 20 which are connected to the time division multiplexed trunk 12 and by connecting, for example, the subscribers A and B with the time division multiplexed trunk 12.
When a return loop is established between the sending and receiving sides of the time division multiplexed trunk 12, the control signal shown in FIG. 2(b) and explained above, is sent from the interface circuit 17 in the sending side and is directly applied to the interface circuit 13 in the receiving side as the waveform shown in FIG. 2(d). Accordingly, when the start signal of channel CH1 is sent at the timing t1 to the remote switching equipment, it is equivalent to transferring a start signal for the same channel CH1 at the timing t1 from the distant exchange. However, since the control circuit 14 performs termination priority processing, transmission of the start signal of channel CH1 to be sent to the time division multiplexed line 19 from the interface circuit 17, is stopped. As a result, the start signal received by the interface circuit 18 is also stopped.
As explained above, if it is required to execute the loop-back test in order to check whether a call is properly terminated to the designated subscriber B from the calling subscriber A only by establishing the return loop connecting the sending side and the receiving side of the time division multiplexed trunk 12, then the loop-back test is impossible. This is because the start signal is transferred back to the time division muliplexed trunk 12 simultaneously with transmission of the start signal to the remote exchange from the time division multiplexed trunk 12.
Therefore, there is a disadvantage in that processing is required to consider the returned start signal as the answer signal in the time division multiplexed trunk 12, so that test processing procedures, which are different from the ordinary exchanging processing procedures must be employed. Thus, there is a need for a time division exchange which is capable of carrying out a loop-back test using normal exchanging processing.